Browsing by Author "Esterhuysen, Catharine"

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    Computational investigation of Au...H hydrogen bonds involving neutral AuI N-heterocyclic carbene complexes and amphiprotic binary hydrides
    (Springer, 2019-04-26) Groenewald, Ferdinand; Raubenheimer, Helgard G.; Dillen, Jan; Esterhuysen, Catharine; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
    In this computational study we investigate the ability of various neutral R-AuI-NHC (NHC = N-heterocyclic carbene) complexes [R = H, CH3, Cl, OH] to form hydrogen bonds with the amphiprotic binary hydrides NH3, H2O and HF. Optimised geometries of the adducts calculated at various levels of theory all exhibit Au···HX hydrogen bonds. In adducts of complexes containing NHC ligands with α -N–H units (NH)carbene···XH interactions also exist, yielding hydrogen-bonded rings with graph-set notation R22(6) that correspond to pseudo chelates with k²C,H coordination. AIM analysis at the MP2/aug-cc-pVTZ-pp level of theory indicates that the (NH)carbene···XH hydrogen bonds are generally stronger than the Au···HX interactions, except for those involving HF. The Au···HX interactions vary with the Lewis basicity of the Au(I) centre as a result of the nature of the R ligand, while the (NH)carbene···XH hydrogen bonds are unaffected by R. Energy Decomposition Analysis at the BP86/TZP level of theory identifies the origin of this difference as the greater component of polarisation involved in Au···HX interactions. Replacing the α(N)Hs with methyl groups prevents formation of a strong (NH)carbene···XH interaction, thus reducing the overall stabilisation of the adducts.
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    Direct determination of enthalpies of sorption using pressure-gradient differential scanning calorimetry: CO2 sorption by Cu-HKUST
    (Wiley, 2020-06-01) Feldmann, Wesley K.; White, Kerry-Anne; Bezuidenhout, Charl X.; Smith, Vincent J.; Esterhuysen, Catharine; Barbour, Leonard J.
    Enthalpy of sorption (ΔH) is an important parameter for the design of separation processes using adsorptive materials. A pressure-ramped calorimetric method is described and tested for the direct determination of ΔH values. Combining a heatflow thermogram with a single sorption isotherm enables the determination of ΔH as a function of loading. The method is validated by studying CO2 sorption by the well-studied metal–organic framework Cu-HKUST over a temperature range of 288–318 K. The measured ΔH values compare well with previously reported data determined by using isosteric and calorimetric methods. The pressure-gradient differential scanning calorimetry (PGDSC) method produces reliable high-resolution results by direct measurement of the enthalpy changes during the sorption processes. Additionally, PGDSC is less labor-intensive and time-consuming than the isosteric method and offers detailed insight into how ΔH changes over a given loading range.
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    Steric and electronic effects in gold N-heterocyclic carbene complexes revealed by computational analysis
    (Wiley, 2019-04-29) Dillen, Jan; Esterhuysen, Catharine; De Kock, Sunel; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
    A computational analysis of a series of cationic and neutral gold imidazolylidene and benzimidizolylidene complexes is reported. The Bond Dissociation Energies of the various ligands in the complexes calculated at the PBE0-D3/def2-TZVP level of theory increase with increasing ligand volume, except for those of complexes containing t-butyl-substituted ligands, which are anomalously low particularly for the benzimidazolylidene species. Atoms in Molecules studies show the presence of a variety of weak intramolecular interactions, characterised by the presence of bond critical points with a range of different properties. Energy Decomposition Analysis and calculation of Electrostatic Surface Potentials indicate that some interactions are weakly attractive dispersion-type interactions, while others are repulsive. The octanol/water partition coefficients (log P values) were calculated as a measure of the lipophilicities of the complexes and were found to increase with increasing volume.